The present invention relates to an oxygen sensor which detects oxygen concentration in the exhaust gas of an internal combustion engine.
A three-way catalyzer is known as one of the effective emission control devices in internal combustion engines. In the three-way catalyzer, HC (hydrocarbon) and CO (carbon monoxide) are oxidized so as to be converted into water and carbon dioxide, and simultaneously, NO.sub.x (nitrogen oxides) is deoxidized so as to be converted into nitrogen gas. In such a three-way catalyzer, in order to achieve an effective catalytic reaction in the catalyzer, the air/fuel ratio of the components in the exhaust gas must be maintained approximately constant at the stoichiometric air/fuel ratio. In order to achieve a precise control of the exhaust gas so that the stoichiometric ratio is maintained, the oxygen concentration in the exhaust gas is detected by an oxygen sensor and, in response to the output signal of the sensor, auxiliary air is supplied to the exhaust gas. Such an oxygen sensor is called an O.sub.2 sensor or a .lambda. sensor.
The sensor element of the oxygen sensor consists of a sintered zirconium dioxide tube closed at one end. The inside and outside surfaces of the tube are coated with a thin layer of platinum. If there are different partial pressures of oxygen on both sides of the tube, electromotive force is generated between the platinum surfaces. The electromotive force steeply changes at the stoichiometric point as shown in the attached FIG. 6 by a solid line "Y". Therefore, the output signal of the sensor element can indicate whether the air/fuel ratio of the gas is richer or leaner than the stoichiometric ratio. Then, auxiliary air is supplied in response to the output signal of the sensor element.
However, in the conventioned sensor element, there is a tendency for the changing point of the electromotive force to be shifted to the lean side, as shown in FIG. 6 by a dot-dash line "Z", due to an oxidizing catalytic characteristic of the platinum coated on the tube surfaces. Such a sensor element indicates the stoichiometric point when the gas is leaner than the stoichiometric ratio. This results in the air/fuel ratio, which is controlled by the sensor element, being shifted to the lean side of the stoichiometric ratio and the three-way catalyzer does not work satisfactorily.
In order to obviate the above drawback, an improved oxygen sensor has already been utilized. This improved sensor comprises a case having an inlet means and an outlet means through which gas passes, a sensor element installed within said case, and an oxidation catalyzer arranged upstream of said sensor element within said case. However, in such an oxygen sensor, the changing point of the electromotive force of the sensor is shifted to the rich side, as shown in FIG. 6 by a dotted line "X", due to the characteristic of the oxidation catalyzer. Such an oxygen sensor indicates the stoichiometric point when the gas is richer than stoichiometric ratio. This results in the air/fuel ratio, which is controlled by the sensor, being shifted to the rich side of the stoichiometric ratio and the three-way catalyzer does not work satisfactorily. In this case, if the oxidation catalyzer has an extremely high catalytic ability which oxidizes over 95% of H.sub.2 and HC in the gas simultaneously, the changing point of the electromotive force of the sensor is not shifted to the rich side. However, such a high catalytic ability can not be maintained over a long period of time and, in addition, such a catalyzer is expensive. Further, a large ammount of catalyzer is needed and the surface of the catalyzer must be widened so as to fully contact the gas, which results is increased resistance against the gas flow.